The invention relates to a three-way catalyst having excellent catalytic activity and aging stability for the purification of exhaust gases from internal combustion engines.
Three-way catalysts are used in large numbers for the purification of exhaust gases from internal combustion engines operated essentially stoichiometrically. They are able to convert the three significant pollutants from the engine, namely hydrocarbons, carbon monoxide and nitrogen oxides, simultaneously into nonpolluting components. Double-layer catalysts which allow a separation of various catalytic processes and thus optimal matching of the catalytic activities in the two layers are frequently used. Catalysts of this type are described, for example, in EP 0 885 650 and EP 1 046 423.
The continually increasing requirements for reducing emissions from internal combustion engines make continual further development of the catalysts necessary. The light-off temperatures of the catalyst for conversion of the pollutants and its thermal stability are of particular importance. The light-off temperature for a pollutant indicates the temperature at which more than, for example, 50% of this pollutant is converted. The lower these temperatures, the earlier after a cold start can the pollutants be converted. At full load, exhaust gas temperatures of up to 1150° C. can occur directly at the outlet from the engine. The better the thermal stability of the catalyst, the closer to the engine can it be located. This likewise improves exhaust gas purification after a cold start.
The catalysts as described in EP 0 885 650 and EP 1 046 423 have very good properties in respect of light-off temperatures and thermal stability. However, the increased legal obligations make it necessary to seek even better catalysts. It was therefore an object of the present invention to provide a catalyst which has further decreased light-off temperatures and improved thermal stability compared to the catalysts of the prior art.